1. Field of the Invention
The invention relates to a quantitative analyzing method by a secondary ion mass spectrometric method and to a secondary ion mass spectrometer. More particularly, the invention is suitable when it is applied to the case of quantitatively analyzing elements of a surface layer of a sample.
2. Description of the Prior Art
A secondary ion mass spectrometric (SIMS) method is an analyzing method having ultrahigh sensitivity characteristics (to ppb level) and high resolution characteristics (to 1 nm) in the depth direction. For example, in the semiconductor field, such an SIMS method is highlighted as an analyzing method which is most useful in case of knowing a concentration distribution of doping impurities in an ultrashallow region due to implantation of ultralow energy ions.
According to the SIMS method, a primary ion beam which has been thinly converged is irradiated onto the surface of a sample to thereby eject out the atoms in the sample, and a part of the ionized particle group, namely, secondary ions are mass analyzed. Generally, as an energy of the primary ions, an energy in a range from a few keV to about 20 keV in which a sputtering yield is high is used. According to the SIMS method, therefore, the analytic region is peeled off one layer by one every time and an ion implantation phenomenon occurs in the depth direction in such a region.
On the other hand, an occurrence probability the secondary ions of a certain special element by the primary ions, namely, a yield of the secondary ions is strongly dominated by a peculiar ionizing potential, a work function of the sample surface, or the like. In other words, the secondary ion yield largely depends on a mother material (matrix) of the sample, coexisting element species, and chemical states of the uppermost surface of a sample, or the like.
In the actual analysis by the SIMS method, molecule-like or atom-like oxygen ions (.sup.32 O.sub.2.sup.+, .sup.16 O.sup.-, etc.) are often used as primary ion species. In this case, however, since the oxygen concentration does not reach an equilibrium state up to a certain depth (critical depth D.sub.x) from the uppermost surface of the sample, the secondary ion yield is also changed every time. Such a phenomenon is called a primary ion implantation effect. Or, such a primary ion implantation effect can be also said as a phenomenon such that the primary ions are implanted into the uppermost layer of the sample and the secondary ion yield is changed every time in dependence on the concentration distribution. In any case, due to the primary ion implantation effect, it is difficult to interpret an intensity of the secondary ions which are obtained in the region of a depth X.ltoreq.D.sub.x from the uppermost surface of the sample. It is, thus, impossible to quantitatively analyze the elements such as impurities or the like in the surface layer of the sample.
Many methods have been proposed so far to solve the above problem.
The first method is a method whereby the secondary ion intensity of a target element which was actually measured is normalized by using the secondary ion intensity of the matrix which was simultaneously actually measured at each depth position and a concentration is calculated on the basis of the normalized secondary ion intensity.
The second method is a method whereby an arbitrary film (in case of a sample of Si, a polycrystalline Si film or an amorphous Si film) having a thickness of 50 to 100 nm is previously formed on the surface of a sample to be analyzed, a region which is influenced by the primary ion implantation effect is shut into such a film, and the secondary ion yield in the region as a target of the analysis is kept constant.
The third method is a method whereby an oxygen gas is purposely introduced into an analyzing chamber of the SIMS at a pressure of up to 10.sup.-3 Pa and an oxygen atmosphere is formed, thereby indirectly eliminating the formation of the region which is influenced by the primary ion implantation effect, and thereby suppressing a change in secondary ion yield due to a depth position and keeping the secondary ion yield constant.
The above first method is effective only in the case where the changing state of the secondary ion yield due to the depth position is equal without depending on the secondary ion species. However, since such an assumption is not generally satisfied, the first method is nothing but a simple convenient method.
According to the above second method, a film cannot always be formed prior to the SIMS analysis and, further, there is a fear of occurrence of a change in impurity distribution due to contamination or thermal disturbance in association with such a film formation.
The third method is, further, not so preferable because when oxygen is purposely introduced into the analyzing chamber, such an introduction will exert an adverse influence on the subsequent analysis.